´It was narrated that Ibn 'Abbas said:` "A man came to him and said: 'I have made my wife forbidden to myself.' He said: 'You are lying, she is not forbidden to you.' Then he recited this Verse: 'O Prophet! Why do you forbid (for yourself) that which Allah has allowed to you.' (And he said): 'You have to offer the severest form of expiation: Freeing a slave.'"
Explanation & Benefits
Hafiz Muhammad Ameen
(1) "You have lied," meaning your declaring your wife forbidden (haram) for yourself is a lie and an incorrect statement, because how can a wife become forbidden? Yes, if it is said with the intention of divorce, that is a different matter.
(2) "The strictest expiation (kaffarah) will be upon you," because you have uttered an extremely vile statement. The wife will not become forbidden, but you will have to bear the punishment for this vile utterance. (See: Hadith 3411)
(3) "To free a slave"—in addition to the Noble Qur'an, feeding or clothing the poor, or fasting, are also mentioned. It is possible that this person was affluent, so Ibn Abbas (radi Allahu anhu) considered it necessary to impose strictness upon him and instructed him to free a slave. And Allah knows best.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 3449